End Fed Half
Wave or "end-fed dipole" Antennas

End-fed halfwave
antennas were once
very popular. Reviewing old antenna books and amateur magazines, longwires and
Zepps were popular antennas. After WWII coaxial lines became readily and cheaply available.
Center fed
dipole antennas appeared, with coaxial feeders that could be easily and safely
routed into the house. Popularity
of end-fed antennas faded as coaxial-fed antennas became mainstream.

End-fed antennas are
increasingly
popular again, at least partly
because of compact iron toroid cores. Small soft-iron cores allow compact, easy-to-build,
low-power transformers and networks. The combination of lightweight compact matching systems,
combined with
the installation convince, visual appeal,
and installation simplicity of NOT hanging a heavy coaxial feeder from a long span of
thin antenna wire, has
rekindled interest in end-fed half-wave antennas.

Unfortunately
end-fed antennas
have also come back with a little misconception. One
commonly repeated
myth or "theory" is
that half-wave
antennas, being
resonant, do not
require a
counterpoise. Lack
of a proper counterpoise does not mean the antenna will be worthless and not
make contacts, it simply means something else replaces the missing counterpoise
area. The feed line, as well as everything connected to and surrounding the
feed line, becomes part of the radiating system. This creates three potential
problems:

1.) The feed line, mast, and things around the feed line connect into the
receiver. This brings noise into the receiver.

2.) The feed line, mast, and things around the feed line become part of the
radiator. This brings voltage (electric fields) and current (magnetic fields)
directly into the shack.

3.) The feed line and grounding affects SWR and tuning.

Since we often do not have a baseline for noise, unnecessary additional noise
will often go unnoticed. The remaining two issues are more likely to be noticed,
but only if we run enough power to cause RF burns, power supply shutdown, or
other forms of RFI.

Transmitter power levels, feed line length and routing, and the
susceptibility of equipment to RF problems greatly influence things we most
likely notice. This is why some people (usually with QRP power levels)
swear by end-fed half-waves, while others (usually with higher power) avoid
end-fed antennas. The reason
for that is simple,
end-fed half waves
have common mode feed line current problems affecting their
performance, and
these common mode currents cause inconsistency
in
user satisfaction. End-fed half wave antennas are best for temporary antennas
using low power and batteries, far from power mains and noise sources. They are
more prone for problems near noise sources or consumer gear, and can easily
exceed FCC RF exposure limits with surprising low power levels.

In nearly all cases, if we notice it or not, an inadequate counterpoise hurts
antenna pattern and efficiency. This is why high power stations often have more
efficient, more ideal, antenna systems. Higher power very often excludes use of
power wasting systems, because the wasted power often creates significant local
problems. If 5% of 10 watts is exciting the desk with RF, it isn't any big deal.
If 5% of 1500 watts excites the desk with RF, the result can be hazardous.

Styles or Types
of EFHW Antennas

There are four
basic styles or
types of end-fed half wave
antennas:

Direct or
traditional feed
antennas, where
the antenna itself
forms the feed line
(inverted L style or longwires)

Coax fed
antenna with a "choke"
or isolator on the
feed line shield
(various articles)

Direct feed
with coax using a lumped
matching network
or tuner at the
end of the antenna
(I-Max 2000, A-99
style, PAR style)

Stub feed with
some sort of open
balanced stub (traditional
Zepp antenna)

Coax feed
with a skirt or
sleeve over the
coax (various
articles)

The five styles
above share one
common trait. Absent
a counterpoise of
some type and proper
care in
feed system construction, the feed methods above
can induce
significant common
mode currents on the
feed system. This
means they are
subject to feed line
radiation and "RF in
the shack", including increased TVI/RFI
and receiving system
local noise pickup.
The actual severity of feed
system common mode
currents in some systems may
surprise many of us, while the lack of problems in other combinations can be
equally surprising.
Low power, long
feed lines, or even
just a lucky length
of feed line or counterpoise can hide common mode transmitting problems, even if
unnecessary
receive noise ingress is more difficult to determine.

Two
configurations,
sometimes called
"end-fed dipoles",
are often the
worse of the end-fed
systems. We'll see
why, as we analyze
each type.

The direct end
feed antenna brings
the end of the
halfwave to a
transformer or
lumped component
network of some
type. The network is
connected to
something it
"pushes"
against so current
can be forced into
the antenna. The
basic inverted-L
antenna system or
"half-wave longwire
antenna" looks like
this:

We can calculate
end-impedance of
this
antenna with Eznec. The
model is:

Wire 1 is the vertical
length, 20 feet.

Wire 2 is the
horizontal length, 46.5 feet

Wire 3 is the small
counterpoise, 5 feet

With 100 watts
applied, current
distribution of the
antenna is:

Wire 2,
horizontal wire 47.5
ft long 24 ft high

dist from 1

1 ft

4.5 ft

9 ft

13 ft

19 ft

23 ft

35 ft

end

current

1.09

1.20

1.25

1.30

1.25

1.16

.71

.02

Wire 1, vertical wire 20 ft long

height

4 ft

8 ft

10 ft

12
ft

16 ft

20 ft

current

.174

.347

.564

.763

.937

1.08

Wire No. 3
counterpoise 5 ft
long 4 ft high:

dist from 1

0 ft

1 ft

2 ft

3 ft

4 ft

5 ft

current

.174

.141

.110

.077

.043

.006

The feedpoint
impedance (the
little circle by the
3 on the
antenna view) is:

The problem with
this system isn't so
much the current and resulting magnetic field
surrounding the antenna
feedpoint. The
dominant problem is the
extreme voltage and
resulting intense
electric field
surrounding the
antenna feedpoint.
With only 100 watts, the antenna feedpoint has several
hundred volts. That voltage substantially increases at the counterpoise's open
end. As a
matter of fact, the
open end of the counterpoise wire
has almost 3,000
volts of peak
voltage with only
100 watts of applied
power!!

One commonly
repeated myth claims end-fed half wave
antennas, being
resonant antennas, do not
require a
counterpoise. We see above, end-impedance is not infinite. In this case it is a
few thousand ohms. Because end-impedance is finite, ground
currents still flow with a
perfectly resonant end-fed.

Changing frequency
10%, which is equivalent
to a 10% antenna length error, we can observe a ground
current increase:

Being 10% off-resonance more than
doubles current. That is a large current increase, and it shows an advantage in
having a resonant half-wave antenna, but even at resonance
end-current is not zero!

Any claim perfect
resonance eliminates
requirements for a
counterpoise is not
correct. A 10%
length error more than doubled
common mode
feedpoint current,
but current was
never zero to start
with. This is
actually common
sense, because the
end impedance of a
1/2 wave antenna is
not infinite. If end-impedance was infinite, we could never manage to apply power. The
end-impedance of any
half-wave is finite,
and varies with
antenna diameter,
length, and
surroundings. Thin
wires have higher
impedance, thick
antennas have much
lower end-impedance, and losses in surroundings tend to further reduce
end-impedance.

Moving 5% in
frequency:

Note:
The very small
difference in
apparent
currents in
the
counterpoise
and antenna at
their junction
is caused by
the
counterpoise
being short.
The short
counterpoise
has a very
rapid
reduction in
current along
its length.
Eznec gives
the mean
current over
the length of
a segment, not
the segment
entrance or
exit current.
This means the
high current
taper makes
the average
current along
the length of
a segment
appear to be
less.

With a 5% length
error from
resonance, we now
see only an 18%
increase in current.
That's negligible
since many other
things we might do
(like moving the
antenna a few feet
in height) would
make a much larger
change.

There
is merit to
maintaining
resonance because
current is at a
minimum value, but
we only need to
worry when resonance
errors are somewhat
large. When length
errors are modest
(under 5%) the error
has only a small effect on ground
current. The reason
for this is
simple. The
reactance or lack of
resonance isn't what
determines current,
the resistive part
of the impedance
does. We are looking
for a resistance
peak in the
end-impedance of the
antenna...not
necessarily
resonance.
The source impedance's resistive
part
was 3300 ohms at resonance. At 5%
length error it was still 2382
ohms. With 10% error, the resistive part of impedance was under 1000 ohms.

With the
non-resonant
antenna, voltage is 925 volts instead of 575 volts. Electric
fields increase around the
feedpoint. RF
voltages (the
electric field)
can be a major issue
with end-fed
longwire antennas, resonant or not.

Very Short or "Missing" Counterpoises

If we truncate a counterpoise, we greatly increase voltage on the counterpoise
and the side of any matching system connected to the counterpoise. Voltage
increases enough to allow the "countering" current to flow into whatever is around
the feedpoint, including the feed line, as displacement or conducted currents, or
a combination of the two.

The fact the feeder becomes involved does not mean the antenna will abruptly stop working. A
seriously truncated counterpoise just means the system is ripe for increased
feed line common mode problems.

Let's look at the feedpoint area of a 40-meter half-wave antenna with a short
counterpoise. In this case the antenna is 66.4 feet long, the "counterpoise"
is 6
feet long, and the feed line (wire 3) is 32 feet long:

The round circle is the source, and the square box represents a 10 pF stray
capacitance from the feeder across the transformer to the counterpoise. We have
the following 100-watt currents:

Wire 1 half-wave horizontal antenna
.16672 amperes

Wire 2 short counterpoise
.08076 amperes

Wire 3 vertically oriented feed line
.06634 amperes

With just 10 pF stray capacitance from primary to the counterpoise side of
the transformer, we have 82% of counterpoise current flowing into the feeder.

Feed impedance seen by the matching transformer secondary in the system above
is:

There is no question this system will radiate, but there is also little doubt
the conductor we assumed was a counterpoise is not the only counterpoise. If we
remove the counterpoise we have the following feed impedance and feed line common
mode:

This shows the extreme
difficulties in isolating the feeder from the antenna when a counterpoise is
very small, even with a half-wave antenna. Just 10 pF of stray capacitance at
the feedpoint results in significant feed line current.

What else
affects Antenna
Impedance?

From above we see
higher antenna
resistance is a good
thing for current,
and precise length is not
overly-critical so far as affecting counterpoise current. We
also see lower
reactance is a good
thing for voltage,
and length can
affect voltages (and
the electric field)
surrounding the
antenna and
counterpoise near
the feedpoint.

What about
a thicker antenna?
With a 1" thick
antenna 7 MHz
impedance becomes
1684 - J 716.3 ohms
and resonance is
well below 6 MHz.
The reactance
problem is because
the counterpoise is
too short. The
drastic resistance
reduction at peak
resistance occurs
because the wire is
thicker. Obviously a
thicker antenna has
higher ground
or counterpoise currents!

(Half-wave
broadcast towers
often have
feed impedances under 800
ohms at
resonance. )

What about
antenna
surroundings? As
the area around the
antenna becomes more
cluttered and/or has
more power loss,
antenna
end-resistance is
reduced! Over
perfect ground the
antenna
end-impedance almost
doubles from that
over average ground.
Over lossy ground,
especially when the
antenna is low in
height, feed
resistance decreases
even more. (End-fed
1/2 wave feed
impedance decreases,
while center
fed
half-wave antenna
impedance increases
with added loss!)

With a
small counterpoise
and end-feed
we need to:

keep the antenna
clear of lossy
media including
the earth.

use a
reasonably thin
antenna element
to minimize
current.

stay within
a few percent of
resonance to
minimize
feedpoint
voltage and
current.

have a good
counterpoise
because we always have
the same current
flowing into a
counterpoise (of
some form) as
flows into the
antenna.

use
the largest
counterpoise
that can be
reasonably
implemented, but
avoid small
counterpoise
systems that
have wires
significantly
longer than 1/4
wl if we can (not possible with multi-band systems)

Longwire Improvements

The best solution
I can think of to
common mode or
"RF in the
shack" problems
with this form of
antenna is to
isolate the
counterpoise or
antenna ground from
the station feed. At
low power levels a
simple link coupled
matching network is
a good solution,
provided the
secondary has no RF
path to station
equipment. One way
to accomplish this
is by using link coupling with two
floating antenna terminals.

U1 goes to
antenna

U2 goes to
counterpoise, do not
connect counterpoise
to station ground

In the circuit
above for a tightly coupled link:

Determine the
maximum
impedance ratio
between input
and
output.

The turns
ratio is the
square root of
that ratio (assuming infinite coupling, which almost never occurs)

The reactance
value of C1 near
3/4 mesh at the
lowest frequency
and secondary
reactance of T1
should be the
maximum expected
load impedance
over the turns
ratio

C2 is
optional, and
should be the
value of C1 or
larger if used.
It will allow
wide adjustment
of matching
range

Assume we have a
5000 ohm load and 50
ohm rig, with perfect mutual coupling between windings. The turns
ratio of T1 is sqrt
of 5000/50 or a 10:1
ratio.

The reactance of
C1 at 3/4 mesh (so
you have adjustment
range) should be
5000/10, or 500
ohms. This is a
loaded Q of ten, you
need LESS Q with
lower transformation
ratios and more Q
with higher ratios
or the circuit
becomes too sharp (with excessive voltages and currents) or
too
"mushy" (we would not be able to correct reactance issues) to have a
good "tune feel".

The reactance of
L1 secondary should
be 500 ohms in this
example.

U1 should connect
to the antenna, U2
to the counterpoise
or ground which
should NOT connect
to the station
equipment. The
counterpoise should
be as long and
straight as
possible, and
directly under the
antenna if possible.
Ideally the
counterpoise, if
less than 8 wires
1/4 wl long, should
be elevated and
insulated from
earth. Do NOT
make the
counterpoise longer
than 1/4 wl,
especially if it is
only a single wire! Most
of us, since this is
a temporary or
compromise antenna,
will use a very
small ground system.I've found
connecting a
counterpoise to
earth, say a ground
rod, actually
reduces antenna
efficiency.

If you run low
power and don't have
a ground or
counterpoise, you
might just connect
U2 right back to the
coax shield from the
radio. This way you
can use the
capacitance of the
radio and station
wiring as a ground
system. There is always some cut and try in this, because mutual coupling is not
perfect and there are stray reactances.

PAR Antenna and
other so-called End-fed
Half-waves or end-fed dipoles

Many antenna
designers and
experimenters assume we can
end-feed a half-wave
without creating potentially
significant feed line common-mode
currents. The antenna
view below shows
common mode currents
in a feed line grounded
3/4 wl away from a
perfectly resonant
end-fed 1/2 wl
antenna:

Currents
are shown below. I
fixed the power
level at 100 watts.
The following are the current
minimums and
maximums:

Maximum antenna current
is .758
amperes, while the
coax common mode
current maximum is
actually 1.02 amperes! It
is actually possible
to have MORE
radiating current at
the common mode current maximum
in the feed line than
in the antenna
itself.

This is because
standing waves on
the OUTSIDE of the
cable transforms
voltage and current.
(This has nothing to
do with standing
waves INSIDE the
feed line that an SWR
meter
measures.)

That was worse
case feed line length. Now here is
best case feed line
shield grounding:

Now we see the
current maximum in
the antenna is 1.12
Amperes, while the
undesired radiation current
maximum in the
feeder is 0.219
amperes. The antenna now
radiates
considerably more
power more than the
feed line. This is a
good thing since we usually want the antenna to radiate more than the feed line!

This problem
doesn't mean the
end-fed antenna won't
work or is a bad
antenna. It does
mean that the
feed line is a part
of the radiating
system, and how the
feeder is routed and
grounded will
drastically affect
system performance.

It is electrically
impossible to
end-feed a 1/2 wl
antenna without a
good counterpoise
at the feedpoint and have
a totally
"cold"
feed line.

A
ten-turn choke or
normal current balun at the
feedpoint certainly
won't change
anything. The only
way to minimize
common mode on the
feed line is to pick
the "lucky feed line
length" or feed line
grounding point. If you don't want the feed line to radiate, center
feed the dipole and use a proper feed line length of a good current balun.